The grades do matter, as they don't just "average out" your results. If you got the equivalent of AAC for an ABB offer, you would not have met that offer so it would be up to the uni whether they accept you on results day as a near miss applicant. Which, they might, but they aren't required to.
That said I would think if you are struggling in a traditionally academic essay subject and doing well in your BTEC course, you may want to consider what degree programme would be best suited to your strengths and not only your interests. Law degrees are usually very academic with a lot of exams.